This invention relates generally to xerographic copying apparatus and, more particularly, to a contact fusing system for fixing electroscopic toner material to a support member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photo-sensitive member or transferred from the member to a sheet of plain paper with subsequent affixing of the image thereto.
In order to permanently affix or fuse electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to be absorbed to some extent into the fibers of the support member which, in many instances, constitutes plain paper. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll to thereby effect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner would liquefy and cause a splitting action in the molten toner to thereby result in "hot offset". Splitting occurs when the cohesive forces holding the viscous toner mass together is less then the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
However, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting) by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the problems attendant the foregoing, particularly that which is commonly referred to as "offsetting" has been to provide a fuser roll with an outer surface or covering of a polytetrafluoroethylene, commonly known as Teflon, to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone based oils, which possess a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to thereby form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface. Additionally, stripping forces for separating the copy sheet from the fuser roll are substantially reduced.
A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of abhesive material to a rigid core or substrate, such as the solid Teflon outer surface or covering of the aforementioned arrangement. The resulting roll structure is subject to degradation due to continued operation at elevated temperatures and also to damage from accidental gouging by stripper fingers conventionally employed in such systems. The foregoing in many instances necessitates replacement of the fuser roll which is quite costly when a large number of machines are involved. Moreover, the initial investment for fabricating such constructions is undesirably high and the manufacturing process is quite cumbersome.
Furthermore, since a several mil thickness of polytetrafluoroethylene along with the coating of silicone oil constitutes a poor thermal conductor, longer nip dwell and higher fuser roll temperatures are required to deliver the fusing energy required. Also, control of the surface temperature of the roll presents a problem due to large temperature variations occurring before and after contacting of the substrate carrying the images.
In view of the foregoing, it would appear that the high thermal conductivity and wear resistance of bare metals or similar materials would be desirable for utilization in fuser roll structures, however, such materials have, heretofore, not been found satisfactory for such application. The latter is attributable to the very high surface energy of metals and similar materials which renders them readily wettable by hot toner materials. Once wetted by hot toner, it has been very difficult if not impossible to remove the toner from such materials while they remain hot. Commonly used release agents such as pure silicone oils have been tried in combination with various metals and other high surface energy materials but with relatively little or no success.
One approach to utilizing bare metal or other high surface energy materials has been to use low molecular weight polyethylene in conjunction with a heated fuser roll structure having a rigid core of copper. While not well understood, it is believed that the polyethylene thermally degrades or oxidizes to form carboxcarbosylic acid which chemically reacts with the surface of the copper core to form a copper-carboxylate layer which forms a barrier preventing toner from contacting the copper core. Unoxidized polyethylene forms a release coating on the copper-carboxylate layer.
It has been found that the foregoing system works well (i.e. stripping forces for separating substrate material from the fuser roll are relatively small) in a black and white system where toner pile heights are relatively small. However, in the process of fixing toner images in a color reproducing apparatus where the toner pile heights are substantially larger than those in black and white systems the stripping portions required to strip the copy paper from the fuser roll structure have been found to be substantial. This has led to re-evaluation of conventional mechanisms for stripping copy paper from the fuser roll structure which traditionally have comprised a plurality of stripper fingers spaced along the longitudinal axis of the fuser roll structure and situated such as to become interposed between the copy paper and the fuser roll structure. As the stripping force required becomes greater the tendency of the stripper mechanism to overcome such forces tends to cause curling of the paper. Attempts at modifying the stripping mechanism have not been totally acceptable.
Accordingly, a primary object of this invention is to provide, in the xerographic reproducing apparatus, a new and improved roll fuser apparatus.
Another object of this invention is to provide, in a roll fuser apparatus, means for facilitating stripping of copy paper from a fuser roll structure, particularly where the fuser is employed for fusing high pile height toner images.
Still another object of this invention is to provide a roll fuser apparatus employing a plurality of release materials which combine to facilitate stripping of copy paper from the fuser roll structure as well as minimizing toner offset to the fuser roll structure.
Yet another object of this invention is to provide in a roll fuser apparatus, means for applying a release material to prevent or minimize toner offset to the fuser roll structure and to facilitate stripping of copy paper from the fuser roll structure wherein release material is applied to the copy paper before it passes through the nip or between the roll structures.